WX BRIEF

Precipitation precautions

There's more to rain and snow than just water falling from the sky.

By Karsten Shein
Comm-Inst, Climate Scientist


Citation 650 negotiates slush-coated taxiway at ALB (Albany NY). Several factors determine whether precipitation falls as rain or snow, but each presents challenges to aviation.

This winter has seen some wicked winter weather across many parts of the Northern Hemisphere's major flight corridors. Blizzard conditions forced the delay or cancellation of thousands of scheduled commercial flights at major airports including ORD (O'Hare, Chicago IL) and SEA (Seattle-Tacoma WA).

Freezing rain coated New England in an ice storm of epic proportions. And this past summer was not much better. A series of June storms flooded much of the Midwest US, while abnormally heavy monsoon rainfall from India to Vietnam disrupted transportation in those areas. Precipitation of any sort tends to have an adverse effect on aviation.

Even a light drizzle can reduce visibility and obscure potentially dangerous obstacles or other aircraft. Despite this, some aspects of precipitation tend to be a mystery to many pilots. For example, if liquid raindrops can exist to temperatures well below freezing, what conditions are necessary to create snow?

When we speak of precipitation, we're generally referring to any falling hydrometeor. Normally that's anything from drizzle to freezing rain, snow, sleet, graupel, ice pellets or hail. Precipitation is usually the end component to the atmosphere's hydrological cycle.

All precipitation starts off as water vapor that has been evaporated or sublimated from water or ice on the Earth's surface. This water doesn't stay in the atmosphere very long-a few seconds to a few days at most-and, despite the amount of rain that may fall from a large thunderstorm, there really is not that much water in the atmosphere at any given time.

If all the water evaporated into the atmosphere were to fall out as rain at the same time, it would cover the earth only to a depth of about 1 inch. But, in its short tenure in the sky, the lowly water vapor molecule can take on a variety of forms and modify its surroundings easily-to the detriment of those of us who must fly through them.

The transitions water makes between solid, liquid and gas are a critical part of the atmospheric system. Water is one of Earth's most abundant and effective elements for absorbing, storing and releasing energy.

It is the absorption of energy from its surroundings that gives the water enough motion to break its bonds with the solid or liquid mass of which it was part. Its ability to store that energy means that it can stay in its new state for a while-often being transported to a new location along with that latent energy.

Temperature profile leading to ice pellets. As snow falls through a layer of above freezing temperature, it partially melts. Cold air below allows it to refreeze into an ice pellet as long as an ice nucleus remains within the liquid drop.

Eventually, it loses the energy to its surroundings and changes state back to a liquid or solid. It is that release of latent energy that drives such things as hurricanes or thunderstorms. The first step in the formation of precipitation is the development of cloud droplets.

Theoretically, a bunch of water molecules can condense at the same time and join together to form a cloud droplet in a process known as homogeneous nucleation. However, in reality, this method of creating a cloud droplet is virtually impossible in the atmosphere because such a droplet instantly creates a situation where the molecules would be evaporated preferentially from it, causing it to vaporize almost as quickly as it formed.

Instead, most cloud droplets condense around an impurity in the air. Fortunately, there is no shortage of atmospheric pollutants-from sea salt to dust, ash and the chemical byproducts of nature and industry-around which condensation may occur.

A cubic meter of the atmosphere can contain hundreds of millions of these microscopic particles, most no larger than 0.02 mm in diameter. That number is much higher in a cloud, where several hundred million cloud droplets on the order of 0.2 mm diameter exist in every cubic meter of atmosphere.

Saturation

(Top)Hexagonal ice crystal with rime accretion viewed through an electron microscope. All ice crystals have a hexagonal lattice and may grow by contact freezing of supercooled liquid droplets. Ice crystal developing a coat of rime ice that will eventually hide the original crystalline structure and result in graupel. Electron microscope view of graupel. The original hexagonal ice crystal inside is completely hidden by rime accretion.

(center) We tend to think of precipitation as something that occurs only when the air is saturated-that is, when the air at a given temperature is holding all the water vapor it's capable of holding. Any extra, we've been taught, must be condensed out as cloud droplets and precipitation. But that explanation is not entirely correct.

(bottom) Under normal atmospheric temperatures there is enough energy available to constantly permit water molecules to transition between states as the energy is being transferred to or from the molecule, even when the air is considered saturated.

Saturation simply means there is a balance between the quantity of water molecules being evaporated and those being condensed. In unsaturated air, evaporation exceeds condensation, while condensation is expected to exceed evaporation in supersaturated conditions.

So, if saturation is a balance between condensation and evaporation, how can a cloud droplet hope to exist for more than a few seconds after its formation, before it re-evaporates into the air? And how can a cloud droplet ever grow to sufficient size to become a falling rain drop? The answer lies in looking at the properties of the droplet itself and the air immediately surrounding it.

Solutions and curvature

Many of the condensation nuclei floating about in the atmosphere are hygroscopic-that is, they attract water preferentially. Salts or clay particles are 2 examples. Think of how difficult it is to pour salt from a shaker on a humid day.

It is these hygroscopic particles that allow a cloud droplet to form even when the air is not saturated (ie, relative humidity is less than 100%). But preferential attraction of water vapor is not enough to keep the droplets from re-evaporating or to allow them to grow.

Instead, the droplets themselves create an environment favorable for continued condensation. Many nuclei, such as salts, dissolve on contact with water, creating a solution. No longer pure water, the droplets have a greater mass than water alone and still have the water-attracting properties of the impurity.

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